The present invention generally relates to submicron photolithography using a phase shifting mask and, more particularly, to a subtractive fabrication method for such a mask employing a sacrificial etch monitor film to control the etch depth of the phase shifting portions of the mask.
Briefly, a phase shifting mask is a transmission mask on which material has been selectively removed or added to change the optical phase of light passing through adjacent apertures. Interference effects between waves from adjacent apertures can lead to enhanced resolution.
It is known in the art that a Levenson type phase shifting mask is very effective to improve lithography resolution for submicron line and space patterns. Such a mask is described in the paper by Levenson et al, "Improved Resolution and Photolithography with a Phase-Shifting Mask", IEEE Transactions on Electron Devices, ED-29, page 1828 (1982).
It is also known that a RIM type phase shifting mask is advantageous for making contact openings and/or isolated space or lines. It is sometimes desirable to include both the Levenson type and the RIM type on the same phase shifting mask to form a hybrid mask. The state-of-the-art methods are either designed for the Levenson type PSM or for the RIM type PSM. None exists that is designed for fabricating a L-R hybrid mask. Types of phase shifting masks are described by P. Burggraaf in Semiconductor International, February. 1992, pp 44-55.
One concern relating to the use of a Levenson type mask is that at the end of the line-space patterns there are residual "loops" caused by phase edge effects of the transmitted waves which link together the line patterns of each adjacent pair of lines. One known solution to this problem is to use a block-out or trim mask to remove the loops. Another known solution is to provide a 90 degree shift transition at the end of the Levenson line-space pairs on the mask. In order to create such a 90 degree shift transition, either an additive or a subtractive method can be employed.
The additive method, however, requires an etch stop film. To assure that the film does not degrade mask performance, several aspects thereof must be considered, e.g., interface effect, film defect density, film refractive index, degree of transmission at the exposure wavelength, etc. The subtractive method avoids the foregoing problems but suffers from another difficulty, namely, that precise etch depth control must be attained to a degree not yet sufficiently demonstrated to make mask fabrication feasible.